Peel and finely chop the onion. Trim and finely chop the celery. Peel and grate or crush the garlic. Warm a medium pan on a medium heat, then add 1 tbsp oil. Add the onion and celery and fry for 8 mins, stirring occasionally, till softened.

While the veg fry, slice the skin off the celeriac. Slice the roots off on the diagonal, leaving as much flesh behind as you can. Give the celeriac a rinse, then chop it into small cubes. Peel and chop the potato in to large chunks. Finely chop the rosemary leaves. Fill and boil the kettle.

After 8 mins, add the garlic to the pan and cook for 1 min, stirring, then add the celeriac, potato and rosemary. Pour in the milk with 500ml hot water from the kettle. Season and pop on a lid. Gently simmer for 20 mins, till the veg are tender.

While the soup simmers, place a small frying pan on a medium heat and add the cashews. Toast them for 3-4 mins, shaking the pan, till golden. Tip onto a chopping board to cool for a few mins, then roughly chop. Set to one side.

Peel and coarsely grate the apple. Peel and finely grate the horseradish (see our heat tip).

When the veg are very tender, add the apple and horseradish to the pan. Blitz the soup till smooth with a hand-held blender (or transfer to a blender to blitz), adding more water if the soup is too thick. Taste and add more salt or pepper if needed.

Ladle the soup into warm bowls and top with the toasted cashews to serve.